The present invention relates to electrodeposited copper foil; to a method of producing the electrodeposited copper foil; and to a copper-clad laminate produced from the electrodeposited copper foil.
Conventionally, copper foil has been employed as a material for producing printed wiring boards, which are widely used in the electric and electronics industries. In general, electrodeposited copper foil is bonded, through hot-pressing, onto an electrically insulating polymer material substrate such as glass-epoxy substrate, phenolic polymer substrate, or polyimide, to thereby form a copper-clad laminate, and the thus-prepared laminate is used for producing printed wiring boards.
In recent years, the thickness, weight, and dimensions of electronic and electric apparatus have been steadily decreasing, and therefore, there is corresponding demand for further downsizing of copper-clad laminates and printed wiring boards which are incorporated into such apparatus. In order to meet the demand, multi-layer technology has been employed to form high density circuits in printed wiring boards.
One important matter which has been pointed out is the application and pattern-formation of a etching resist, which is to be performed before etching to form circuits, so as to obtain fine printed wiring boards having a circuit pitch of 50 xcexcm to 80 xcexcm. Briefly, in order to achieve the required aspect ratio of the circuit formed by etching copper foil, a resist layer is appropriately formed without a problem before etching.
Several attempts involving etching process have been made to obtain printed wiring boards having a fine circuit pitch. Such attempts include (1) employment of resists such as a liquid resist and an electrodeposited resist instead of a dry film resist so as to reduce the thickness of the resist layer for sufficient supply of an etchant to the portions to be etched; (2) specifically roughening a copper foil surface through polishing such as physical polishing; e.g., buffing or micro etching so as to prevent dropping and adhesion-failure at the interface between the resist layer and the copper foil, thereby enhancing adhesion at the interface; and (3) selection of etching conditions such as the type of etchant in accordance with an etching machine, and the method of showering an etchant.
However, for copper foil to be etched during the production of printed wiring boards, only one countermeasure has been considered; i.e., further reducing the thickness of the copper foil to shorten the etching time and to avoid lowering the aspect ratio of the circuit obtained from the copper foil.
Particularly, adhesion between the copper foil and the resist material is a phenomenon manifested at the interface of an organic material and an inorganic material. Few studies have been conducted on the above phenomenon, particularly on the mechanism thereof, as compared with other technical fields. Thus, this form of adhesion has not yet been fully elucidated.
During production of printed wiring boards, an etching resist layer is formed on a copper foil; the layer is exposed so as to form a circuit(wiring) pattern; and the exposed layer is developed so as to remove the unneeded portions of the resist. However, when printed wiring boards having a circuit pitch of 50 xcexcm to 80 xcexcm are actually produced, some portions of the resist layer required to remain adhered onto the copper foil is sometimes unintentionally removed. For this reason, adhesion between the resist layer and the copper foil has been considered an important issue.
For example, even though the portion of the resist layer which has been dropped away is very narrow, the portion causes a continuity failure; i.e., an open circuit may be created during etching to form the circuit, thereby considerably reducing the production yield of printed wiring boards.
Even when the circuit obtained from a copper foil has no open circuit after etching, a decrease in the width of the circuit obtained during the etching process may cause problems. Briefly, when the thus-produced printed wiring boards are inspected and even no open circuit is found, there may still be a narrowing of the circuit at certain portions, resulting in increased electric resistance of the narrower portion. If such a printed wiring board is incorporated into an electric apparatus and operated, the high-resistance portion generates a lot of heat, thereby causing considerable deterioration of the copper circuits and rapid, unfavorable hardening of the wire. As a result, an open circuit may be induced by the wiring board being subjected to a low-intensity shock, possibly leading to a sudden breakdown of the electric apparatus.